1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine.
2. Related Background Art
Conventionally, there have been known a color image forming apparatus which is a so-called in-line system in which a plurality of photosensitive drums are arranged in tandem. In this image forming apparatus, while a transfer material is borne and conveyed by an electrostatic transfer belt stretched around and tensioned by a plurality of rollers, toner images of yellow, magenta, cyan and black are sequentially transferred onto the transfer material by four photosensitive drums arranged along a conveying path of the transfer material. A color image is formed by superimposing the respective colors.
This structure has been recently a focus of attention since a printing operation can be performed at high speed. However, since the respective colors are formed by the four photosensitive drums, further accuracy is required with respect to a rotation driving of the photosensitive drums in comparison with a color image forming apparatus constructed such that the colors are superimposed via four conveying paths every color with respect to a single photosensitive drum (hereinafter simply referred to as xe2x80x9cfour path systemxe2x80x9d).
Namely, a gear train is generally adopted to drive the photosensitive drum, and an uneven rotation at low frequency in one revolution component of the gear, etc. is inevitably generated. However, in the case of the four path system, an accumulative pitch error of the gear, etc. are avoided by setting a reduction gear ratio of the driving gear train to a combination of integers, and image forming positions of the respective colors can be registered with each other.
However, in the case of the in-line system, the plural photosensitive drums are independently arranged. Therefore, the driving gear trains are also independent and it is difficult to take the above avoiding measure as in the four path system. Therefore, there is a fear that color misregister is caused by faulty registrations of the image forming positions of the respective colors.
Accordingly, an equal accuracy or more is required with respect to a banding caused by engagement of the gear etc.
The banding will next be explained. When line images having a small equal intervals in a sub-scanning direction and drawn in a direction perpendicular to a conveying direction of the transfer material, an image of a medium density, or the like are printed, the image is formed by changing the interval of the line images, dots, etc. when a rotation speed of the photosensitive drum is periodically changed. The density looks dense in a narrow interval area, and looks light in a wide interval area. Accordingly, it is recognized in the human eyes as a gradual uneven density corresponding to the periodic change of the rotation speed of the photosensitive drum.
Such a defect in the image is called a banding, an uneven pitch, or a regular striped pattern. The period from about 0.3 mm to several mm is conspicuous in human visible sensitivity. Therefore, when the gear train is used in the driving of the photosensitive drum, the banding corresponding to the periodic change in the rotation speed of the photosensitive drum is easily caused among defects caused by the engagement every one tooth of the gear and the uneven rotation every one revolution.
A gear coupling system and a coaxial coupling system are conventionally adopted as a means for transmitting a rotation driving force from a driving source arranged on an apparatus main body side to the photosensitive drum. In the gear coupling system, a driving gear on the apparatus main body side connected to the driving source is connected to and disconnected from a gear coaxially fixed to the photosensitive drum. In the coaxial coupling system, a driving side coupling on the apparatus main body side is axially moved and is connected to and disconnected from a driven side coupling coaxially fixed to the photosensitive drum.
When the gear coupling is adopted in the above conventional example, a structure is simple and the cost therefor can be reduced.
In contrast to this, when the coaxial coupling is adopted, the gear train can be completed on the apparatus main body side. Therefore, there are advantages in that the banding is easily reduced by using a gear having a diameter larger than an outer diameter of the photosensitive drum and a small module, etc.
The present invention has been made by further developing the above conventional art.
An object of the present invention is to provide an image forming apparatus in which a rotation accuracy of a photosensitive drum is improved.
Another object of the present invention is to provide an image forming apparatus in which a prevention of an image banding and a prevention of a color misregister are improved.
Another object of the present invention is to provide an image forming apparatus comprising:
a first engaging member fixed to a rotary shaft of a photosensitive drum;
a second engaging member in which an engaging portion capable of engaging with the first engaging member, a positioning hole capable of fitting onto the rotary shaft to support the rotary shaft, and a shaft portion fitted into a bearing portion arranged in an apparatus main body to be rotatably supported by the bearing portion, and a gear connected to a driving source are integrally formed; and
urging means for urging the second engaging member against the first engaging member in an axial direction of the rotary shaft of the photosensitive drum;
wherein under a state in which the shaft portion of the second engaging member is fitted into the bearing portion arranged in the apparatus main body to be rotatably supported by the bearing portion and the rotary shaft is fitted into the positioning hole of the second engaging member to be supported by the positioning hole, the engaging portion of the second engaging member is engaged to the first engaging member to connect therebetween by urging force of the urging means in the axial direction whereby a rotary axis of the second engaging member is positioned.
In accordance with the above structure, the shaft portion of the second engaging member is fitted into the bearing portion arranged in the apparatus main body, and is rotatably supported by the bearing portion. The rotary shaft of the photosensitive drum is fitted into the positioning hole of the second engaging member, and is supported by the positioning hole. Thus, a positioning for aligning the second engaging member with an axial direction of the photosensitive drum can be performed in two positions on an apparatus main body side and a photosensitive drum side. Accordingly, a rotary axis center of the second engaging member can be reliably aligned with the center of rotation of the photosensitive drum.
Thus, a change in angular velocity of the photosensitive drum etc. is prevented, and its rotation accuracy can be improved by a simple structure. Accordingly, an image defect such as a banding and a color misregister can be prevented, and the cost therefor can be reduced.
These and other objects, features and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.